Can you use an Isolation transformer as an Automatic transformer?

I am currently working on an Isolation transformer that was designed to have 600V on the primary side and either 480V or 208V (depending on the taps you come off of) on the secondary side. My problem is that this transformer is actually being used to step 480V down to 208V. Basically only using the secondary side of the transformer. Functionaly this works and I get the 208V I need but I am not sure how this set-up affects the transformer and if it changes it's rating. (rated as a 1500KVA transformer).

Does anyone know how using an isolation transformer like this will affect it's rating and what other possible problems there might be?


To further clarify...

Yes, it is indeed a 1500kva transformer. The application for this is that I am using the transformer with a 500kW 480V diesel generator and need to transform the voltage down to 208V. With the transformer I am feeding the 480V into the 480V tap on the secondary side of the transformer and pulling 208V off the 208V tap on the secondary side. So like the first answer assumed, I am not using the primary side of the transformer at all.

I get my desired voltage but I am worried about a) how Ontario's electrical authority (ESA) will view this and b) possible damage to the tranformer.

I understand that I will no longer have isolation between primary and secondary but I am hoping to find some way of figuring out what the KVA rating of the transformer will be reduced to when only using the secondary side.

5 Answers

  • 8 years ago
    Favorite Answer

    The transformer is limited to 650 KVA when used as an autotransformer. In other words you are limited to a maximum load of 3125 A (1804 A if it's 3-phase) on the 208 V tap. Here's why:

    Think of it this way. The Xfrmr has a rating of 1500 kVA meaning that the primary coil has an ampacity of 2500 A (1443 A if it's 3-phase). The secondary coil has an ampacity of 3125 A (1804 A if it's 3-phase).

    The secondary coil ampacity doesn't depend on whether you're using the 480 V tap or the 208 V tap. If you're using the 208 V tap, you are still only allowed a maximum of 3125 A, which gives the 208 V tap a rating of 650 kVA. I know it may seem odd to de-rate the 208 V tap, but if you incorrectly assume that the 208 V tap is rated at 1500 kVA, you'll find that you're also assuming the coil has an ampacity of 7,211 A. This is impossible since we already established the coil has an ampacity of only 3125 A. The ampacity of a wire doesn't depend on the applied voltage. The insulation will begin breaking down due to heat once you surpass the maximum current.

    In theory, connecting the seconday coil as an autotransformer will actually INCREASE its KVA rating (from 650 KVA). This is becasue the "primary" current and the "secondary" current cancel each other out in the mutual winding. If you make a quick schematic sketch of an autotransformer and apply Kirchoff's Current Law at the node where the 208 V tap is, you will see that if you assume the "primary" is at maximum current (3125 A) and the mutual winding is at maximum current (3125 A), they will add together to make the "secondary" current 6250 A (3608 A if it is 3-phase). This means the actual KVA rating of your transformer is 1300 KVA when used as an autotransformer.

    In reality though, even though the secondary coil can handle a 1300 KVA load as an autotransformer, the internal wiring and terminal lugs you connect to cannot. It would be safe to assume they can only handle the 3125 A for which they were originally designed. This means that the autotransformer should be run at a maximum of 650 KVA. In other words, don't draw more than 3125 A (1804 A if it is 3-phase) off the 208 V tap in the autotransformer configuration.

    That being said, you should verify that your installation meets the CSA Canadian Electrical Code standards (Rule 26-266 Autotransformers):

    (1) In this Rule, "auto-transformers" means transformers in which part of the turns are common to both primary and secondary alternating-current circuit.

    (2) Auto-transormers shall not be connected to interior wiring systems, other than a wring system or circuit used wholly for motor purposes, unless

    (a) the system supplied contains an identified grounded conductor solidly connected to a similar identified grounded conductor of the system supplying the auto-transformer;

    (b) the auto-transformer is used for starting or controlling an induction motor; or

    (c) the auto-transformer supplies a circuit wholly within the apparatus that contains the auto-transformer.

    (3) Where an auto-transformer is used for starting or controlling an induction motor, it may be included in a starter case or it may be installed as a separate unit.

    (4) Notwithstanding Subrule (2), auto-transformers shall be permitted for fixed voltage transformation in circuits not incorporating a grounded circuit conductor.

    I would over-stepping my bounds if I ventured to interpret CEC rules for you, as that should only be done by an inspection authourity. If you want more help with the code rule, phone your inspector and ask. They are usually helpful if you're putting the effort in to be compliant.

    Source(s): B.Sc. Electrical Engineering 4th year Electrician Apprentice (Alberta) CSA CEC Part II 2012 Schaum's Outlines Basic Circuit Analysis Second Edition (for a good explanation of auto-transformers if you're interested)
  • 8 years ago

    I think you mean "autotransformer".

    I'm assuming you are using the secondary as an autotransformer, and have no connection to the primary at all. (A lot of these answers make a different assumption)

    You will get NO isolation using it this way, and the VA rating would be reduced. How much it is reduced, can't tell. I'd use half as a start. No isolation means the input and output have to share a common wire, which would have to be the neutral.

    and yes, 1500kVA is a huge transformer. if you use the full rating, at 208 volts that is 7000 amps. Probably room size. Perhaps you mean 1500 VA, which is 7 amps?



    A transformer this size, you can contact the manufacturer and get the exact numbers you need. But you only need 500 kVA out of 1500 kVA, so there should be no problem, you are using only 1/3 the capacity.

    As to how the power company will see this, why would they see it at all? UNLESS you are connecting the 208 volts to the power company lines, which would result in a huge problem.

    Small phase differences would cause huge currents and blow out fuses and breakers all over the place. If you want to feed power to the power company, you need special controls on the generator that they would approve. The controls match frequency and phase and adjust voltage to get the correct current flow and direction.

  • pliw
    Lv 4
    8 years ago

    Automatic transformer (also known as auto transformer) is not an isolation transformer. Auto transformer has only one winding. The full winding ( primary side)connects to the source and tapping along this winding is your output or secondary. Isolation transformer are those that have two separate winding. One winding for the primary and the other winding for the secondary. There is no physical connection between the two winding hence called isolation transformer. The primary is isolated to the secondary.

    Connecting an auto transformer with 600V primary to a 480V source to produce 208V will not have much effect on the rating. What matter is if you exceed the 1500KVA rating.

  • Anonymous
    8 years ago

    You say it was designed to have 600V on the primary but that is not the actual voltage on the primary.

    Transformers are in turns ratios. When you keep increasing the primary voltage so does the secondly voltage goes up proportionally providing the primary winding and the core saturation can handle it.

    1500KVA is what the transformer can handle so your output will be function of V * A = VA. So if you series the secondary windings you get double the voltage at half the current. If you try and draw more current the voltage will drop. You just need to take into account the winding thickness and what current you plan to draw.

    1500KVA are you sure???? That's a massive transformer! That's like 3125 Amps at 480V! Do you mean 1500VA?

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  • 8 years ago

    Since you are not using the primary, you need to derate to about 1200 KVA. Generally if the transformer does not get hot you are ok. Your connection is called an autotranformer and it is possibly a code violation, and a slight safety hazard in some failure modes. Neil

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